PRECAST CEMENT CONCRETE BLOCKS CONSTRUCTION

The mould should be either fixed type or (box with four side walls fixed at corners, and top and bottom open) or split type.

Split type may be either individual or gang mould. Where the compaction of the concrete is done manually, the mould may be either fixed type or split type. When the composition of the compaction of the blocks is done with surface vibrator, the mould shall be only split type (individual or gang mould).

Demoulding shall be done 5 to 10 minutes after compaction. In case of fixed type mould it shall be pulled up with one side handles while pressing down the blocks with the place at top with thumb. In case of split mould, the sides shall be removed first and the partition plates (gang mould) shall be pulled up subsequently.

After demoulding, the blocks shall be protected until they are sufficiently hardened to permit handling without dam

2 Proportion:-The normal proportion of the mix shall be as specified. To attain maximum strength, the water-cement ratio and workability of the mix should or controlled and proper compaction of concrete in the mould ensure age.

Fig.1 Concrete Block Mould

3 Casting:- After mixing, the concrete shall be placed in the moulds immediately, being carried in metal gamelas other suitable vessels or by a chute.

The concrete should be mixed in a concrete mixer and the water-cement ratio should be the smallest should be preferably by the use of a small immersion vibrator or table vibrator.

4 Curing:- The blocks hardened shall then be cured in a curing water tank or in a curing yard and shall be kept continuously moist for at least 14 days.

5 Drying:- After curing, the blocks shall be dried for a period of two to four weeks depending upon weather before being used on the work. The blocks shall be allowed to complete their initial shrinkage before they are laid in a wall.

Fig.2 Curing Tank For Concrete Block

6 Dimensions:-Concrete masonry building units shall be made in sizes and shapes to fit different construction needs. They include stretcher, corner, double corner or pier, jamb, header, jamb, bull nose, and partition block, and concrete floor units.

Concrete block hollow (open or closed) or solid shall be referred to by its nominal dimensions.

The nominal dimensions of concrete block shall be, as follows:

Length : 400,500 or 600 mm

Height : 200 or 100mm

Width : 50, 75, 100, 200, 250 or 300mm

in addition, block shall be manufactured in half lengths of 200, 250 or 300mm to correspond to the full lengths.

Fig.3 Architectural Concrete Block

7 Tolerances:-The maximum variation in the length of the units shall not be more than +5mm and maximum variation n height and width of unit, not more than +3mm.

8 Density:- The hollow blocks shall be provided cavities in such a way so as to ensure the maximum block density of 1600 kg/m3. The block density is the density is the density calculated by dividing the mass of a block by the overall volume including holes or cavities.

CONSTRUCTION OF MASONRY

For single storeyed buildings,the hollows of blocks in foundation and basement masonry shall be filled up with sand and only the top foundation course shall be of solid blocks. But for two or more storeyed buildings,solid concrete blocks shall be used in foundation courses, plinth, and basement walls, unless otherwise indicated. If hollow blocks are used, their hollows shall be filled up with cement concrete 1:3:6 using 12.5 mm nominal size aggregates.

Fig.4 Concrete Block Wall

1 Wetting of Blocks:- Blocks need not be wetted before or during laying in the walls. In case the climate conditions so require, the top and the sides of blocks may only be slightly moistened so as to prevent absorption of water from the mortar and ensure the development of the required bond with mortar.

2 Laying:-Blocks shall be laid in mortar,as indicated and thoroughly bedded in mortar, spread over the entire top surface of the previous course of blocks to a uniform layer of not less than 10 mm and not more than 12mm in thickness .

All course shall be laid truly horizontal and vertical joints made truly vertical Blocks shall break joints with those above and below for not less than quarter of their length .Precast half length closer and not cut from full size blocks shall be used .For battered face ,bedding shall be at right angle to the face unless otherwise directed .

Care shall be taken during construction to see that edge of block are not damaged.

3 Provision for Door and Window Frames:-A course of solid concrete block masonry shall be provided under door and window openings (or a 10 cm thick precast concrete sill block under windows). The solid shall extend for at least 20 cm beyond the opening one either side .For jambs very large doors and windows either solid units are used, or the hollows shall be filled in with concrete of mix 1:3:6 using 12.5 mm nominal size aggregates.

Fig.5 High Concrete Block Walls Of Mansion

Fig.6 Precast Concrete Fencing Wall

4 Intersecting Walls:- when two wall meet or intersect and the course are to be laid up at the same time , a true masonry bond between at least 50% of the units at the intersecting is necessary. when such intersecting walls are laid up separately, pockets with 20mm maximum vertical spacing shall be left in the first wall laid .The corresponding course of the second wall shall be built into these pockets.

5 Provisions for roof:-The course immediately below the roof slab shall be built with solid blocks .The top of the roof course shall be finished smooth with a layer of cement and coarse sand mortar 1:3,10 mm thick and covered with a thick coat of white wash or crude oil , to insure free movement of slab.

6 Piers:-The top course of block in the pier shall be built in solid blocks. Hollow concrete block shall not be used for isolated piers, unless their hollows are specified to be filled with a cement concrete.

Fixtures,fitting ,etc. shall be built into the masonry in cement and carse sand mortar 1:3 while laying the blocks where possible .Hold fasts shall built into the joints of the masonry during laying .

Holes, chases, sleeves , openings etc. of the required size and shape shall be fprmed in the masonry with special blocks while laying,for fixing pipes,service lines ,passage of water etc. After service lines ,pipes,etc. are fixed ,voids left ,if any , if any , shall be filled up with cement concrete 1:3:6 (1 cement :3 coarse sand :6 stone aggregate 2 mm nominal size ) and neatly finished .

3.7 Finishes:-Rendering shall not be done to the walls when walls are wet . Joints for plastering or pointing as specified shall be raked to a depth of 12 mm. joints on internal faces, unless otherwise indicated ,shall be raked for plastering . if the internal faces of masonry are not to be plastered the joints shall be finished flush as the work proceed or pointed flush where so indicated

ADVANTAGES AND DISADVANTAGES

ADVANTAGES:-

1. Concrete of superior quality is produced as it is possible to have better technical control on the production of concrete in factory.

2. It is not necessary to provide joints in pre-cast construction.

3. The labour required in the manufacturing process of pre-cast units can easily be trained.

4. The moulds employed for preparing the pre-cast units are of steel with exact dimensions in all directions. These moulds are more durable and they can be used several times.

5. The pre-cast articles may be given the desired shape and finish with accuracy.

6. The pre-cast structures can be dismantled ,when required and they can be suitably used elsewhere.

7. The transport and storage of various components of concrete for cast-in-situ work are eliminated when pre-cast members are adopted.

8. The work can be completed in a short time, when pre-cast units are adopted. When pre-cast structures are to be installed , it is evident that the amount of scaffolding and formwork is considerably reduced.

9. When pre-cast structures are to be installed , it is evident that the amount of scaffolding and formwork is considerably reduced.

DISADVANTAGES:-

1. If not properly handled, the pre-cast units may be damaged during transport.

2. It becomes difficult to produce satisfactory connections between the pre-cast members.

3. It is necessary to arrange for specific equipment for lifting and moving of pre-cast units.

4. The economy achieved in pre-cast construction is partially balance by the amount to be spent in transport and handling of pre-cast members. It becomes, therefore, necessary to locate the pre-cast factory at such a place that transport and handling charges are brought down to the minimum possible extent.